Ultraviolet observations of mass-loss effects in O stars have, over the past decade, revealed a broad picture of a phenomenon whose extent was only partially evident from earlier ground-based observations. Ultraviolet resonance lines of a variety of ionization stages of several common elements provide a comprehensive probe of the low-density, extended winds. Three general types of information have been derived from ultraviolet spectroscopy of mass-loss profiles: (1) the nature of the stars which experience mass loss via radiatively-driven winds; (2) the physical conditions in the winds; and (3) variability in the outflow, which in turn may yield clues to the origins of the winds. Observations and results in each of these areas are reviewed, and some new results are included. A good correlation of mass loss rate and luminosity is indicated by the data, in agreement with theoretical predictions. Time variations in the P Cygni profiles may be quite common, with variability on times of hours or longer. Anticipated new observations, which should be possible with existing and planned instrumentation, are described.